Silver halide photography has been practiced for more than a century. The radiation sensitive silver halide compositions initially employed for imaging were termed emulsions, since it was not originally appreciated that a solid phase was present. The term "photographic emulsion" has remained in use, although it has long been known that the radiation sensitive component is present in the form of dispersed microcrystals, typically referred to as grains.
Over the years silver halide grains have been the subject of intense investigation. Although high iodide silver halide grains, those containing at least 90 mole percent iodide, based on silver, are known and have been suggested for photographic applications, in practice photographic emulsions almost always contain silver halide grains comprised of bromide, chloride, or mixtures of chloride and bromide optionally containing minor amounts of iodide. Up to about 40 mole percent iodide, based on silver, can be accommodated in a silver bromide crystal structure without observation of a separate silver iodide phase. However, in practice silver halide emulsions rarely contain more than about 15 mole percent iodide, with iodide well below 10 mole percent being most common. All silver halide grains, except rarely employed high iodide silver halide grains, hereinafter excluded from consideration except as expressly noted, exhibit cubic crystal lattice structures.
It has been recognized for many years that the ratio of silver halide grain surface area to grain volume is not constant. Finer silver halide grains exhibit higher grain surface area in relation to grain volume, more commonly referred to indirectly in terms of coating coverages--e.g., grams of silver per square meter. An increased ratio of silver halide grain surface area to grain volume, hereinafter referred to as the grain surface area ratio, can be advantageous in improving photographic performance dependent on surface effects, such as interaction with processing agents as well as interactions with adsorbed addenda, such as spectral sensitizing dyes.
However, extremely fine grain emulsions, such as Lippmann emulsions, which have the highest surface area ratios, are not commonly employed for forming latent images in silver halide emulsions, since they exhibit low photographic speeds. Within the range of silver halide grain sizes normally encountered in photographic elements the maximum speed obtained at optimum sensitization increases linearly with increasing grain size. Thus, radiation sensitive emulsions have often represented a compromise between meeting photographic speed objectives dictating larger grain sizes and satisfying other performance criteria benefiting by increasing grain surface area ratios and therefore favoring finer silver halide grains.
A variety of regular and irregular grain shapes have been observed in silver halide photographic emulsions. While grains can show corner and edge rounding attributable to a lower activation energy for silver halide solubilization at these locations, in general silver halide grains are polyhedral, being bounded by distinct crystal faces.
Silver halide favors the formation of crystallographic faces of either the cubic or octahedral form. Silver chloride strongly favors the formation of cubic crystal faces. Silver bromide also favors the formation of cubic crystal faces, but favors the formation of octahedral crystal faces in the presence of an excess of bromide ions. Iodide ions in the crystal structure tend to increase the grain preference for crystal faces of the octahedral form. A discussion of the factors which cause one crystallographic form to be favored over another is offered by James, The Theory of the Photographic Process, 4th Ed., Macmillan, New York, 1977, pp. 98 through 100.
Regular silver halide grains bounded by cubic crystal faces are cubic in appearance when examined by electron microscopy. A regular cubic grain 1 is shown in FIG. 1. The cubic grain is bounded by six identical crystal faces. In the photographic literature these crystal faces are usually referred to as {100} crystal faces, referring to the Miller index employed for designating crystal faces. While the {100} crystal face designation is most commonly employed in connection with silver halide grains, these same crystal faces are sometimes also referred to as {200} crystal faces, the difference in designation resulting from a difference in the definition of the basic unit of the crystal structure. Although the cubic crystal shape is readily visually identified in regular grains, in irregular grains cubic crystal faces are not always square. In grains of more complex shapes the presence of cubic crystal faces can be verified by a combination of visual inspection and the 90.degree. angle of intersection formed by adjacent cubic crystal faces.
The practical importance of the cubic crystal faces is that they present a unique surface arrangement of silver and halide ions, which in turn influences the grain surface reactions and adsorptions typically encountered in photographic applications. This unique surface arrangement of ions as theoretically hypothesized is schematically illustrated by FIG. 2, wherein the smaller spheres 2 represent silver ions while the larger spheres 3 designate bromine ions. Although on an enlarged scale, the relative size and position of the silver and bromide ions is accurately represented. When chloride ions are substituted for bromide ions, the relative arrangement would remain the same, although the chloride ions are smaller than the bromide ions. It can be seen that a plurality of parallel rows, indicated by lines 4, are present, each formed by alternating silver and bromine ions. In FIG. 2 a portion of the next tier of ions lying below the surface tier is shown to illustrate their relationship to the surface tier of ions.
In another form regular silver halide grains when observed by electron microscopy are octahedral in appearance. A regular octahedral grain 5 is shown in FIG. 3. The octahedral grain is bounded by eight identical crystal faces. These crystal faces are referred to as octahedral or {111} crystal faces. Although the octahedral crystal shape is readily visually identified in regular grains, in grains of more complex shapes the presence of octahedral crystal faces can be verified by a combination of visual inspection and the 109.5.degree. angle of intersection formed by adjacent octahedral crystal faces.
Ignoring possible ion adsorptions, octahedral crystal faces differ from cubic crystal faces in that the surface tier of ions can be theoretically hypothesized to consist entirely of silver ions or halide ions. FIG. 4 is a schematic illustration of a {111} crystal face, analogous to FIG. 2, wherein the smaller spheres 2 represent silver ions while the larger spheres 3 designate bromine ions. Although silver ions are shown at the surface in every available lattice position, it has been suggested that having silver ions in only every other available lattice position in the surface tier of atoms would be more compatible with surface charge neutrality. Instead of a surface tier of silver ions, the surface tier of ions could alternatively be bromide ions. The tier of ions immediately below the surface silver ions consists of bromide ions.
In comparing FIGS. 1 and 2 with FIGS. 3 and 4 it is important to bear in mind that both the cubic and octahedral grains have exactly the same cubic crystal lattice structure and thus exactly the same internal relationship of silver and halide ions. The two grains differ only in their surface crystal faces. Note that in the cubic crystal face of FIG. 2 each surface silver ion lies immediately adjacent five halide ions, whereas in FIG. 4 the silver ions at the octahedral crystal faces each lie immediately adjacent only three halide ions.
Five remaining achievable crystallographic forms for cubic crystal lattice materials are not favored by silver halide. In a few instances silver halide grains having faces of the rhombic dodecahedral form have been observed. Crystal faces of the rhombic dodecahedral form in silver chloride and silver chlorobromide emulsions are reported by Claes et al U.S. Pat. No. 3,817,756. Wyrsch, Papers from the 1978 International Congress of Photographic Science, Rochester, N.Y., II-13, p. 122, reported rhombic dodecahedral silver chloride emulsions prepared by a triple jet precipitation procedure in the presence of divalent cadmium ions and ammonia. Berry, "Surface Structure and Reactivity of AgBr Dodecahedra", Photographic Science and Engineering, Vol. 19, No. 3, May/June 1975, pp. 171 and 172, illustrates silver bromide grains having crystallographic faces of the rhombic dodecahedral crystallographic form.
A regular rhombic dodecahedral grain 7 is shown in FIG. 5. The rhombic dodecahedral grain is bounded by twelve identical crystal faces. These crystal faces are referred to as rhombic dodecahedral or {110} (or, less commonly in reference to silver halide grains, {220}) crystal faces. Although the rhombic dodecahedral crystal shape is readily visually identified in regular grains, in grains of more complex shapes the presence of rhombic dodecahedral crystal faces can be verified by a combination of visual inspection and measurement of the angle of intersection of adjacent rhombic dodecahedral crystal faces.
Rhombic dodecahedral crystal faces can be theoretically hypothesized to consist of alternate rows of silver ions and halide ions. FIG. 6 is a schematic illustration analogous to FIGS. 2 and 4, wherein it can be seen that the surface tier of ions is formed by repeating pairs of silver and bromide ion parallel rows, indicated by lines 8a and 8b, respectively. In FIG. 6 a portion of the next tier of ions lying below the surface tier is shown to illustrate their relationship to the surface tier of ions. Note that each surface silver ion lies immediately adjacent four halide ions.
There are four additional crystallographic forms which can be exhibited by cubic crystal lattice structures, but which have never been reported previously for silver halide. These are the hexoctahedral, tetrahexahedral, trisoctahedral, and icositetrahedral crystal forms. Silver halide grains having faces of these crystal forms are the subject of concurrently filed, commonly assigned U.S. Ser. Nos. 771,861, 772,228, 772,229, and EMULSIONS WITH NOVEL GRAIN FACES (1), (2), (3), or (4), respectively.
The seven possible crystallographic forms for cubic crystal lattice structure materials are named for the polyhedrons that are produced by a regular crystal structure bounded entirely by faces of a single crystallographic form. For example, regular silver halide grains bounded entirely by crystallographic faces of the cubic form are cubes; bounded entirely by crystallographic faces of the octahedral form are octahedra; etc.
In addition to regular grains of a polyhedral shape produced by being bounded entirely by crystal faces of the same crystallographic form, it is not uncommon to observe regular silver halide grains bounded by both cubic and octahedral crystal faces. Such grains are referred to as being cubo-octahedral. This is illustrated in FIG. 7, wherein cubo-octahedral grains 9 and 10 are shown along with cubic grain 1 and octahedral grain 5. The cubo-octahedral grains have fourteen crystal faces, six cubic crystal faces and eight octahedral crystal faces, and for that reason they are sometimes alternatively referred to as tetradecahedral grains. Analogous combinations of cubic and/or octahedral crystal faces and rhombic dodecahedral crystal faces are possible, a rare example of grains having cubic, octahedral, and rhombic dodecahedral crystal faces being provided by Berry, cited above in connection with rhombic dodecahedral grains.
Further diversity in silver halide grain shape can be attributed to irregularities in the grains, such as twin planes or screw dislocations. Irregular grains of distinctive shapes, often observed in minor proportions, such as tabular silver bromide grains having octahedral crystal faces, have been the subject of many silver halide crystallographic studies. Klein et al, "Formation of Twins of AgBr and AgCl Crystals in Photographic Emulsions", Photographische Korrespondenz, Vol. 99, No. 7, pp. 99-102 (1963) describes a variety of singly and doubly twinned silver halide crystals having cubic and octahedral crystal faces. Klein et al is of interest in illustrating the variety of shapes which twinned silver halide grains can assume while still exhibiting only cubic or octahedral crystal faces.
Recently dramatic photographic improvements have been obtained with thin as well as high aspect ratio tabular grain emulsions, as illustrated by Wilgus et al U.S. Pat. No. 4,434,226; Kofron et al U.S. Pat. No. 4,439,520; Daubendiek et al U.S. Pat. No. 4,414,310; Abbott et al U.S. Pat. Nos. 4,425,425 and '426; Wey U.S. Pat. No. 4,399,215; Solberg et al U.S. Pat. No. 4,433,048; Dickerson U.S. Pat. No. 4,414,304; Mignot U.S. Pat. No. 4,386,156, Mignot Research Disclosure, Vol. 232, August 1983, Item 23210; Jones et al U.S. Pat. No. 4,478,929; Maskasky U.S. Pat. No. 4,400,463; and Wey et al U.S. Pat. No. 4,414,306. Research Disclosure is published by Kenneth Mason Publications, Ltd., Emsworth, Hampshire P010 7DD, England. While thin and high aspect ratio tabular grain emulsions exhibit high surface area ratios, their major faces are of the same cubic or octahedral crystallographic forms exhibited by silver halide grains of other shapes.
There has been some investigation of silver halide grains of composite shapes produced by depositing silver halide either of the same or a different composition onto a host silver halide grain.
Core-shell silver halide emulsions constitute the most common examples of silver halide grains of a composite structure. Core-shell emulsions are illustrated by Porter et al U.S. Pat. Nos. 3,206,313 and 3,317,322, Berriman U.S. Pat. No. 3,367,778, and Evans U.S. Pat. No. 3,761,276, and, in tabular form, by Evans et al U.S. Pat. No. 4,504,570.
Turning to composite silver halide grains in which the additionally deposited silver halide does not form a shell around the host silver halide grains, Koitabashi et al U.S. Pat. No. 4,349,622 discloses epitaxially depositing on silver halide grains containing from 15 to 40 mole percent iodide silver halide which contains less than 10 mole percent iodide.
Hammerstein et al U.S. Pat. No. 3,804,629 discloses that the stability of silver halide emulsion layers against the deleterious effect of dust, particularly metal dust, is improved by adding to physically ripened and washed emulsion before chemical ripening a silver chloride emulsion or by precipitating silver chloride onto the physically ripened and washed silver halide emulsion. Hammerstein et al discloses that silver chloride so deposited will form hillocks on previously formed silver bromide grains.
Berry and Skillman, "Surface Structures and Epitaxial Growth on AgBr Microcrystals", Journal of Applied Physics, Vol. 35, No. 7, July 1964, pp. 2165-2169, discloses the growth of silver chloride on silver bromide. Octahedra of silver bromide form growths all over their surface and are more reactive than cubes. Cubes react primarily at the corners and along the edges. Twinned tabular crystals form growths randomly distributed over their major crystal faces, with some preference for growths near their edges being observed. In addition, linear arrangements of growths can be produced after the emulsion coatings have been bent, indicating the influence of slip bands.
Maskasky U.S. Pat. No. 4,435,501 teaches high aspect ratio tabular grain emulsions having one or more silver salts deposited at selected surface sites. Maskasky U.S. Pat. No. 4,463,087 is essentially cumulative, but additionally discloses deposition at the edges and corners of nontabular silver halide host grains. Each patent teaches the use of adsorbed site directors to locate silver salts at selected sites on the host grains.
A. P. H. Trivelli and S. E. Sheppard, The Silver Bromide Grain of Photographic Emulsions, Van Nostrand, Chapters VI and VIII, 1921, is cited for historical interest. Magnifications of 2500X and lower temper the value of these observations. Much higher resolutions of grain features are obtainable with modern electron microscopy.
W. Reinders, "Studies of Photohalide Crystals", Kolloid-Zeitschrift, Vol. 9, pp. 10-14 (1911); W. Reinders, "Study of Photohalides III Absorption of Dyes, Proteins and Other Organic Compounds in Crystalline Silver Chloride", Zeitschrift fur Physikalische Chemie, Vol. 77, pp. 677-699 (1911); Hirata et al, "Crystal Habit of Photographic Emulsion Grains", J. Photog. Soc. of Japan, Vol. 36, pp. 359-363 (1973); Locker U.S. Pat. No. 4,183,756; and Locker et al U.S. Pat. No. 4,225,666 illustrate teachings of modifying silver halide grain shapes through the presence of various materials present during silver halide grain formation.
Wulff et al U.S. Pat. No. 1,696,830 and Heki et al Japanese Kokai No. 58[1983]-54333 describe the precipitation of silver halide in the presence of benzimidazole compounds.
Halwig U.S. Pat. No. 3,519,426 and Oppenheimer et al, "Role of Cationic Surfactants in Recrystallization of Aqueous Silver Bromide Dispersions", Smith Particle Growth and Suspension, Academic Press, London, 1973, pp. 159-178, disclose additions of 4-hydroxy-6-methyl-1,3,3a,7-tetraazaindene to silver chloride and silver bromide emulsions, respectively.
F. C. Phillips, An Introduction to Crystallography, 4th Ed., John Wiley & Sons, 1971, is relied upon as authority for the basic precepts and terminology of crystallography herein presented.